examples/ipsec-secgw: define initial ESN value

examples/ipsec-secgw: define initial ESN value

New option added to the SA configuration arguments that
allows setting an arbitrary start value for ESN.

For example in the SA below ESN will be enabled and first egress
IPsec packet will have the ESN value 10000:

sa out 15 cipher_algo null auth_algo null mode ipv4-tunnel \
src 172.16.1.5 dst 172.16.2.5 \
esn 10000

Signed-off-by: Declan Doherty <declan.doherty@intel.com>
Signed-off-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Akhil Goyal <gakhil@marvell.com>